To be able to offer telecommunications services at zero tariff, the feasibility and associated acceptance of commercial spots in telephone calls was examined in a test (see Telefongesellschaft Otelo under http://www.userpage.fuberlin.de/·dittbern/Telekom/Old_News5.html). For example, the Berlin Phone Company TeleFlash planned to interrupt conversations every 90 seconds in order to insert a 20 second long commercial. However, in exchange, the conversations were to be free of charge. (See Golem Network News of Jan. 18, 1999 http://www.ggn.de/9901/9901 1803-ji.html). For these approaches, contact information is specified in the advertisement which the customer must note if he/she is interested. After ending his conversation, the customer can contact the advertising partner in question via this contact address.